Human skin may contain a range of abnormalities including vascular and pigmented lesions. Although not always dangerous to the individual, such abnormalities are frequently cosmetically troublesome.
Vascular lesions, in particular, may take several manifestations. Common examples are `port wine` stain birthmarks; telangiectasias (spots or vessel lines formed by dilated capillaries or other small blood vessels); and hemangiomas (benign tumors composed of well-formed blood vessels). Pigmented lesions generally consist of hyperactive melanocytes which produce a local overabundance of melanin.
Leg telangiectasia, or `leg veins`, are chronically dilated blood vessels visually apparent as red or blue linear or `spider` structures. They may cover extensive or local areas of the leg and are more common in women. Large diameter vessels may cause discomfort, while smaller diameter vessels are more often considered cosmetically unsightly by patients.
Up to 80 million adults in the United States alone are affected by leg veins. It is estimated that 29-41% of women and 6-15% of men worldwide have `abnormal` (visually apparent) leg veins. Most vessels presenting for treatment are less than 1 mm in diameter although candidates for treatment have diameters up to 3 mm.
The vessels consist of dilated blood channels in an otherwise normal dermal stroma. The blood channels have a single endothelial cell lining with thickened walls consisting of collagen and muscle fibers. Clinically, these vessels may be categorized as linear, arborizing, spider or papular.
Such dilated vessels may result from pregnancy or the use of progestational agents. A genetic link is usually also present. Some such veins are associated with a high pressure flow from a feeding reticular or varicose vein.
In order to eradicate a leg vein, it is usual to damage the endothelial vessel lining or surgically ligate the vessels. Such surgery is radical and performed on an in-patient basis. Endothelial damage may be induced by means of Sclerotherapy or by the use of light energy on an outpatient basis.
Sclerotherapy is currently the favored method of non-surgical leg vein eradication.
Sclerosing agents have traditionally been employed to damage endothelial cells. Such agents as sodium tetradecyl sulfate, hypertonic saline and polidocanol are injected into large vessels (&gt;1 mm in diameter) and result in death of the endothelium. Several systemic injections to a `feeder` vessel system may result in widespread death of the ectatic vessels.
The use of sclerosing agents is associated with telangiectatic matting (formation of clusters of small vessels) in 35% of patients treated, and with hyperpigmentation (residual brown pigmentary staining) in up to 30% of vessels treated. Other adverse sequelae are possible, including ulceration, edema (blistering) and systemic anaphylactic shock. Vessel recurrence within 5 years has been observed in up to 40% of patients studied. Further, many patients are fearful and resistant to the use of needles.
Hyperpigmentation pursuant to sclerotherapy is particularly troublesome, as it replaces the blue vessels with a brown discoloration which may persist for up to 5 years. This effect results from the catabolism of extravasated blood to hemosiderin, a form of iron deposition, brown in color, which may reside in the proximal dermis for up to 6 months.
Sclerotherapy injection difficulties render sclerotherapy relatively unsuitable for the routine treatment of vessels with diameters of less than 1.0 mm and for the treatment of many larger vessels with diameter in the range 1.0-3.0 mm.
Light energy has been utilized for the treatment of cutaneous vasculature.
When use of light is under consideration, one can choose to vary wavelength, pulsewidth or coherence (uniformity). Wavelength will typically be chosen by consideration of the absorption and scattering characteristics of the target tissue layers. The absorption characteristics are typified by several peaks in the visible region of the spectrum, due to target chromophores, together with a monotonic decrease into the infra-red region. The scattering of tissue decreases monotonically through the visible to the near infra-red region and beyond.
Both coherent laser light and incoherent light from a flashlamp-type source offer the potential for high selectivity of treatment. Short wavelength (&lt;500 nm) light is usually not employed, since it is highly scattered in tissue and therefore unable to penetrate to a sufficient depth. Light of a wavelength greater than 500 nm has been employed for the treatment of vascular lesions. The absorption profile of whole blood is shown in FIG. 1. This profile will vary with anatomical location, since blood constitution varies, but can be taken as generally representative.
Vascular diseases characterised by small vessels such as the Port Wine Stain respond well to visible wavelength pulsed laser light from a pulsed dye laser, typically with a wavelength in the 550-600 nm range, which is tuned to a local absorption peak of the intra-vascular blood. Such light, which is absorbed in the top 0.05 mm of the vessel, can coagulate and thereby thrombose a significant portion of the entire cross section of small vessels (&lt;0.1 mm). Construction of such a pulsed dye laser for dermatology applications has been described previously.
Visible wavelength laser light is less effective on larger diameter vessels (&gt;0.1 mm). The main reason for this is that it is too highly absorbed in blood. Although vessel rupture is possible, this represents a non-optimal mechanism associated with the involvement of only the superficial portion of the vessel, due to the shallow absorption depth of the light. Regrowth of the insufficiently damaged vessels usually occurs under these circumstances. Also, the rupture of the vessel leads to an unsightly post-treatment purpura (`bruising) which can persist for up to 2 weeks. This is not well tolerated by patients.
It should be remembered also that dilated vasculature of the extremities is also associated with a different and variable ratio of oxy/deoxygenated hemoglobin, the main absorbing chromophores within the blood. Different considerations are then pertinent in devising an appropriate therapeutic regime. A typical leg vein is characterised by a relatively low oxygenation of around 70%, responsible for an occasional blue `hue` in some vessels. (Hemoglobin, as typically found in port wine stains on the face, is bright red in color and usually approximates a constant 95-100% oxygenation level). The near infra-red absorption characteristics of the two hemoglobin types which dominate blood absorption are shown in FIG. 2. Both hemoglobin types have equal absorption around 800 nm, rendering absorption independent of chromophore mix (and hence of anatomical location) at this wavelength. This provides a useful insensitivity to anatomical location and individual characteristics in terms of precise level of oxygenation. The magnitude of the absorption coefficient around 810 nm is well suited to the dimensions of the target vessels. Light at this wavelength is absorbed in a 2 mm blood layer, as opposed to light in the historically employed 500-600 nm region, which is absorbed in a blood thickness of less than 200 .mu.m.
Short wavelengths are also highly scattered as they pass through the turbid dermis to reach the target vessels. An increase in scattering of more than 50% occurs as wavelength is shortened from the near infra-red to the mid-visible. This renders light in the 500-600 nm region less suited to the targeting of deeper dermal vessels.
A further disadvantage associated with existing short wavelength coherent laser sources such as the pulsed dye laser is their short pulsewidth. With a maximum around 1.5 milliseconds, no time for concurrent conduction of the heat is permitted. Further, such an exposure interval is better suited to the thermal relaxation time constants of overlying melanocytes, leading to unwanted temperature rise and the possibility of damage. Such melanocytes have thermal relaxation time constants in the range 100-300 .mu.secs, and would retain significant thermal energy within a 1.5 millisecond exposure. An available pulsewidth of up to several tens of milliseconds would be desirable and would obviate this effect.
Also, the high cost and the significant bulk of the componentry associated with short wavelength (500-600 nm) coherent light sources are prohibitive factors.
A broadband-emitting incoherent flashlamp light source has been suggested to offer an alternative approach for the treatment of leg veins. Such a source may utilize a spread of principally infra-red wavelengths (550-1200 nm) most of which exhibit a smaller degree of absorption better suited to larger vessels. A longer pulsewidth of up to 100 milliseconds is also available, permitting concurrent heat conduction through the vessel and beyond to a radius of up to 250 .mu.m. As a consequence, the full volume of the vessels may be affected, as required for vascular necrosis, although significant perivascular necrosis may result.
Clinical results from the use of this class of source are at the preliminary stage and may include a reduction of the hyperpigmentation associated with the shorter wavelength/pulsewidth dye laser since proximal rupture is no longer the mechanism in effect. Adverse effects include the occurrence of gross heating effects, edema and blistering associated with the incoherent light, since incoherent light has poor penetration characteristics in human tissue. Also, the broad mix of wavelengths includes spectral regions which are less suited to the lesion characteristics, such as the 1000-1200 nm region, which displays little vascular selectivity.
Further, such a system is physically clumsy and difficult to use.
Such an incoherent light cannot be easily focused to a spot size which efficiently overlaps the vessels and hence unaffected tissue is involved in the pathological effects.
The above disadvantages, taken together, limit the applicability of this technology.
Another manifestation of the incoherent flashlamp based light source relates to the use of a mercury-xenon vapor lamp, with specific emission peaks in the visible portion of the spectrum. This incoherent source will often result in gross heating of proximal tissue, with a resultant need for concurrent cooling of the skin. Further, the visible emission spectrum of the lamp lends its use to small vessels found in Port Wine Stains, since the light will be absorbed in the top 0.05 mm of the vessels. Larger leg veins are not cited in the patent for this device for this reason.
FIG. 3 illustrates graphically the effect of tuned visible (.about.580 nm) and near infra-red (700-900 nm) coherent light on small (&lt;0.1 mm) and moderate (0.1 mm&lt;diameter&lt;1.0 mm) sized vessels. This figure illustrates the inherent suitability of visible band light to small vessels and of infra-red band light to moderately sized vessels, since destruction of a significant proportion of the vessel is required. This suitability is fully harnessed only if pulse widths of the order of several tens of milliseconds are available, with their concurrent conduction permitting useful proximal vessel wall damage. In particular, the aforementioned pulsewidth of 1.5 milliseconds likely will not permit sufficient conduction of heat to guarantee vascular elimination, since a radius of only 30 .mu.m is reached in this time. Such a short pulsewidth will further threaten the overlying epidermal layer. Also, extravasation and secondary purpura and hyperpigmentation are likely since efficient coagulation of the intravascular blood and extravascular tissue rim is not attained.
While such short pulses may be appropriate for very small vessels (&lt;100 .mu.m) which lose heat rapidly, larger vessels are likely to require proportionately longer exposures. Vessels with size in the range 100-500 .mu.m may require exposure time intervals of 1.5-40 milliseconds, while vessels larger than 500 .mu.m may require exposure time intervals of 1.5-100 milliseconds.
In FIG. 3, the shading shows the heat generation during the pulse resulting from direct absorption. The denser shading associated with visible light signifies the attainment of high localized temperatures with associated explosive effects. This heat can be expected to conduct further to affect a peri-vascular tissue volume if sufficiently long pulse widths are employed.
A near infra-red narrow-band coherent laser light source with variable pulsewidth would offer the potential for more thorough coagulation of larger vessels, without the adverse effects attendant with the flashlamp source. Such a source intentionally employs wavelengths which exhibit lower blood specificity, contrary to traditional approaches where maximum specificity is sought. Further, such a source would better penetrate to the required depth in tissue than do visible light wavelengths or incoherent infra-red wavelengths, since optical scattering is comensurately reduced. An alternative source of near infra-red light and associated treatment method is described in the following sections.